P-boryl ketones

Very recently, Ma and coworkers reported another bicyclic triazolium ligand 102 based on [2.2]paracyclophane and its applications in the asymmetric copper (I)-catalyzed p-borylation of a,p-unsaturated acyclic enones (Scheme 3.64) [93]. The catalyst generated in situ by the reaction of 102 and CU2O was remarkably efficient giving the p-boryl ketones in high yields (up to 99%) and enantioselectivities (up to 97%). A screening of the ligand revealed that the absolute... 

The imidazolium salt L36 is also effective when acyclic, aryl- or alkyl-substituted enones are used as substrates. The corresponding linear p-boryl ketones can be obtained from 56 to 94% yield and >98% ee (Scheme 76). 

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

Kleeberg et al. [113] have proven the existence of the diboron NHC compound B2pin2 (pin = OCMe2CMe20), an intermediate previously postulated by Hoveyda and coworkers to be generated during the NHC-catalyzed P-borylation of a,p-unsaturated ketones [114]. 

Scheme 8 Synthesis of p-borylated N-acyloxazolidinones and ketones with paracyclophane-derived NHC-copper(I) catalysts introduced by Song...

Bis(pinacolato)diboron and bis(neopentyl glycolato)diboron have been added to a,P-unsaturated ketones, esters, nitriles, and aldehydes through [RhCl(PPh3)3] activation, presumably via oxidative addition. However, Nishiyama has suggested a bond metathesis step to form Rh(III)-boryl species, from B2pin2 and [Rh(III)(OAc)2(OH)(Phebox)] complex, which... 

Notwithstanding the extremely rich and diverse reactivity presented above, all the examples presented thus far feature the addition of an NHC catalyst onto an sp carbon. Several reactions have been developed in the last ten years describing the activation of heteroatom o bonds by NHCs [8cj. The concept can be well illustrated by the recent work of Hoveyda and coworkers, who reported boryl and silyl conjugate additions to a,P-unsaturated carbonyls (Scheme 18.35) [124]. In both cases, addition of the NHC onto a trivalent boron atom activated the B-heteroatom bond. The use of Cj-symmetric imidazolinium salt 169 in the sUyl conjugate addition led to P-silyl ketones, esters, and aldehydes in good to high enantioselectivity. 

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